Give it a really good shake every five minutes. It will all have dissolved by the third time you shake it, if not sooner. A huge amount of salt and sugar (maltodextrin, dextrose, honey, brown rice syrup, etc) will dissolve in water.

Agree with that. It doesn't work as well if the water is cold, though -- in other words, refrigerate after mixing, don't start with chilled water.

I've diluted a half-cup of malto in an 8-oz. plastic water bottle with far less than 8 oz. of water. It took a lot of shaking, but now I have a homebrew energy gel flask, of sorts.

I think for the electrolyte potion, I'll mix up a concentrated batch to store in the fridge and dilute it into bike bottles as I go. Maybe throw in a squirt of fruit juice for flavoring so it doesn't taste like I'm drinking my own sweat.

Seems promising. When I was looking around I could not find any complete caps like those at my local shops. The Vitamin C probably adds to the cost a bit and isn't really necessary, but certainly can't hurt. Having each serving in a cap like that would also make it easier to carry around extras. Good find!